The president calls it a “witch hunt” on Twitter, the White House calls it “Russia fever” – but Robert Mueller, who has now spent exactly one year looking into Russian meddling during the 2016 presidential campaign, shows no signs of slowing down in his secretive investigation that looms over the Trump White House.

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On May 17 of last year, acting Attorney General Rod Rosenstein tapped Robert Mueller, who served more than a decade as director of the FBI, to investigate “any links and/or coordination between the Russian government and individuals associated with the campaign of President Donald Trump” and “prosecute federal crimes arising from the investigation of these matters.”

Since his appointment, Mueller and his team of hand-picked prosecutors have interviewed dozens of witnesses, issued eight indictments covering 19 individuals and three businesses, secured five guilty pleas, have two criminal cases headed to trial, and one individual already serving a prison sentence.

The Targets

Mueller's highest-profile targets: Paul Manafort, who served as Donald Trump’s campaign chairman, and former Lt. Gen. Michael Flynn, the president’s first national security adviser.

Manafort, 69, faces two criminal cases related to the special counsel’s charges of money laundering and tax fraud, among a laundry list of other charges that predate his work on Trump’s campaign. Both of Manafort’s cases are scheduled to go to trial, starting with his case in Virginia which is slated to begin July 10. None of the special counsel’s charges against Manafort is connected to his time working for then-candidate Trump.

Of the five individuals who have pleaded guilty to charges filed by the special counsel, the most consequential may be Flynn – the only cooperating witness to have served in President Trump’s White House.

Flynn was one of Trump’s earliest campaign surrogates and gained notoriety with Trump supporters for leading “lock her up” chants, referring to Hillary Clinton, on the campaign trail and at the Republican National Convention.

A decorated soldier with a 33-year military career, Flynn was removed from his post as national security adviser in February 2017 after less than a month on the job for what the White House initially said was misleading Vice President Mike Pence about his conversations with Russia's ambassador. Flynn pleaded guilty in December to one count of lying to FBI agents and awaits sentencing scheduled for later this summer.

Rick Gates, a former Trump campaign aide and business associate of Paul Manafort, is another high-level witness who pleaded guilty earlier this year to charges including conspiracy against the United States.

Gates, who – like Manafort – was charged in both Washington, D.C., and Virginia, has agreed to “cooperate fully, truthfully, completely, and forthrightly” with the special counsel’s office as part of his plea deal. After pleading guilty, the special counsel dropped charges in Gates’ Virginia case, though he still faces the possibility of serving time in prison for charges in Washington, D.C.

Both Flynn and Gates have been part of ongoing debriefing sessions with Mueller’s prosecutors as the special counsel continues its probe.

In a February indictment, Mueller charged 13 Russian nationals and three Russian businesses for meddling “with U.S. elections and political processes” in what Mueller’s prosecutors describe as a large-scale social media campaign meant to sow discord and distrust among Americans during the 2016 presidential election.

Last week, an attorney hired by one of the Kremlin-linked companies indicted by Mueller pleaded not guilty on their client’s behalf to the special counsel’s charges.

Mueller’s other indictments have been of peripheral figures like George Papadopoulos, a foreign policy adviser on the Trump campaign who pleaded guilty in October 2017 for lying to federal investigators; Richard Pinedo, a California man who pleaded guilty to charges of identity fraud in February 2018 for his role in unwittingly selling bank accounts to Russians; and Alex van der Zwaan, a Dutch attorney who pleaded guilty in February 2018 for lying to federal investigators about his contacts with Rick Gates.

Van der Zwaan is already serving his 30-day sentence in a minimum-security federal prison in Pennsylvania, according to data from the Federal Bureau of Prisons – the first prison sentence related to the special counsel’s investigation.

Papadopoulos’ status on the campaign has become fodder for critics of the Mueller investigation and a source of disagreement among former Trump campaign officials. Trump initially named Papadopoulos as one of his campaign’s foreign policy advisers, calling him an “excellent guy” during an interview with The Washington Post editorial board.

But after Papadopoulos came under scrutiny from the special counsel, advisers to the president sought to diminish Papadopoulos’ role in the campaign, with one of Trump’s former political advisers, Michael Caputo, going so far as to call Papadopoulos a “coffee boy.”

After Papadopoulos’ guilty plea, President Trump tweeted that “Few people knew the young, low-level volunteer named George, who has already proven to be a liar,” referring to Papadopoulos.

In addition to the individuals Mueller has already indicted, the special counsel appears to have interviewed a prolific number of witnesses, including over 20 White House officials, a smattering of former campaign staffers, and a number of foreign entities.

Earlier this month, ABC News reported exclusively that Mueller’s team has questioned several witnesses about millions of dollars in donations to Trump’s inauguration committee last year, including questions about donors with connections to Russia, Saudi Arabia, the United Arab Emirates and Qatar.

Detractors of Mueller’s investigation have argued that Mueller has waded too far from his chartered mission of investigating Russian meddling during the 2016 campaign.

In court, Manafort has filed motions dismiss both of his criminal cases on grounds that Rosenstein and Mueller have both gone beyond their legal duties in assigning a special counsel and in charging Manafort with crimes seemingly unrelated to his work on Trump’s 2016 campaign. Manafort also filed a civil suit against the Justice Department for similar reasons, though that case was thrown out in April.

In a Virginia courthouse earlier this month, a federal judge expressed similar skepticism of the special counsel’s broad scope, questioning Mueller’s prosecutors during a hearing about Manafort’s aforementioned motion to dismiss. Earlier this week, a federal judge in the Washington case threw out Manafort's latest challenge to Mueller's authority.

Trump vs. Mueller

In the White House, President Trump has indulged in slamming the special counsel’s work over the past year, repeatedly framing Mueller’s investigation as a “witch hunt” on Twitter and elsewhere. Aides to the president have called for an expedient end to the special counsel’s investigation, most recently with Vice President Mike Pence calling on Mueller to “wrap up” his work.

But after a year working this case, it’s clear Mueller’s probe is far from over – and there’s one high-profile witness the special counsel is still hoping to interview: President Trump.

As ABC News has previously reported, the special counsel remains focused on sitting down with President Trump in some capacity. After FBI agents raided the office of Trump’s longtime personal attorney, Michael Cohen, sources close to the president say he’s “less inclined” to agree to an interview with Mueller, and it appears tension between the White House and special counsel is now at an all-time high.

Evan Vucci/AP

President Donald Trump listens during a roundtable on immigration policy in California, in the Cabinet Room of the White House, May 16, 2018, in Washington.

During a lengthy rant in front of the reporters at the White House in April on the same day as news of the raid on Cohen came out, the president accused the special counsel’s team of being the most “conflicted group of people I have ever seen.”

Trump has shuffled his legal team several times, now with former prosecutor and New York City Mayor Rudy Giuliani taking the helm in negotiations with the special counsel.

Earlier this month, Giuliani told ABC News’ George Stephanopoulos that he can't rule out the possibility of the president taking the Fifth Amendment if he testifies in the Russia investigation, adding that "we don't have to" comply with a subpoena from Robert Mueller.

But the hostilities don’t stop there.

At various points over the past year, sources have told ABC News that President Trump has toyed with the idea of firing Mueller or his Justice Department overseer, Deputy Attorney General Rod Rosenstein.

Last month, asked directly about whether he's determined to fire Mueller and Rosenstein, the president declined to say.

"They’ve been saying I’m going to get rid of them for the last three months, four months, five months. And they’re still here," Trump said.

As vocal as the president has been about the special counsel’s investigation – just this week calling it a “$10,000,000 Russian Witch Hunt” – Mueller’s office has maintained its stoic posture, rarely offering reporters more than a “no comment” and only releasing official statements to announce indictments or plea deals.

Mueller appears focused on completing his ultimate task with as little fanfare as possible: complete his investigation and present the Department of Justice with a report on his findings.